1. Field of the Invention
The present invention relates to an inkjet printing head used in a printing apparatus for ejecting printing solutions such as inks from ejection ports to print.
2. Description of the Related Art
In recent years, machines such as computers, facsimile machines and copiers have spread widely. And, various printing methods have been developed and used in these machines. Among other things, an inkjet printing apparatus adopting an inkjet printing method in which inks are ejected on a printing medium for printing has excellent characteristics, that is, it is easier in providing high-accuracy printing than those according to other printing methods, capable of printing at high speed quietly and also lower in price.
The above-described inkjet printing apparatus is provided with a printing head having ejection ports for ejecting inks. Then, known methods for ejecting inks include a method in which an electromechanical converter such as a piezoelectric element is used to eject inks and a method in which an electrothermal converter such as a heating resistor is installed to heat inks, thereby causing film boiling to eject inks due to the action thereof.
FIG. 8 is a view showing a conventional inkjet printing cartridge (hereinafter, simply referred to as a printing cartridge as well). A printing cartridge 601 is constituted by forming an inkjet printing head unit containing a printing element substrate 702 made with silicon or the like integrally with an ink container unit 709 containing inks there inside. The printing element substrate 702 is provided with a heater for ejecting inks by converting electric energy to thermal energy. The printing element substrate 702 is constituted with a substrate having a wiring for transmitting the electric energy supplied from an inkjet printing apparatus to the heater, channels for supplying inks to the heater and a nozzle plate having a plurality of ejection ports for ejecting inks. Then, one printing element substrate 702 is provided with ejection port rows 703, 704, 705 for ejecting three color inks, that is, yellow, magenta, and cyan. An electric wiring substrate 706 is to transmit an electric signal from the inkjet printing apparatus to the printing element substrate 702, transmitting the electric signal from the inkjet printing apparatus via an external signal input terminal 707. The electric wiring substrate 706 is electrically connected to the printing element substrate 702 at two end faces of the printing element substrate 702, and the electrically connected part is covered with a sealant 708 and protected from the inks.
FIG. 9 is a drawing showing a supporting substrate 802 for supporting the printing element substrate 702 and a supporting plate 804 for fixing and supporting the electric wiring substrate 706. The supporting substrate 802 is made with a material such as alumina for bonding and fixing the printing element substrate 702 with high accuracy and also subjected to polishing. The supporting plate 804 is also made with a material such as alumina, as in the case of the supporting substrate 802.
FIG. 10 is a cross-sectional view taken along the line X to X in FIG. 8. The supporting substrate 802 is provided with ink supply ports 803 for supplying inks inside the ink container unit 709 to the printing element substrate 702, and the printing element substrate 702 is installed so that the above-described ink supply ports 803 are communicatively connected to the ink supply ports 803 of the printing element substrate 702. Further, a supporting plate 804 is installed so as to enclose the periphery of the printing element substrate 702. A sealant 805 is placed between the printing element substrate 702 and the supporting plate 804 to seal them, thereby preventing inks from entering between the printing element substrate 702 and supporting plate 804. If no sealant 805 is used for this purpose, an ink will enter between the printing element substrate 702 and the supporting plate 804 and the ink will adhere on a side face end of the printing element substrate 702. Silicon is exposed at the side face end of the printing element substrate 702, and there is a case where the silicon will leak out when the ink adheres on the side face end thereof.
The sealant 805 is used to seal between the printing element substrate 702 and the supporting plate 804, thus making it possible to prevent leakage of silicon. Further, it is because an electrically connected part is protected from an ink that the sealant 805 is used to seal between the printing element substrate 702 and the supporting plate 804. As the sealant 805, there is generally used a thermosetting resin which can be handled relatively easily in production processes.
The accuracy on installation of the printing element substrate 702 directly influences the printing accuracy of an inkjet printing apparatus. Thus, in order to increase the accuracy on installation thereof or increase a yield in production processes, various proposals have been so far made. Japanese Patent Laid-Open No. H10-044420 (1998) has proposed that in fixing a printing element substrate, a supporting substrate substantially equal in thermal characteristics to the printing element substrate be bonded to fix the printing element substrate. Further, Japanese Patent Laid-Open No. 2002-019119 has proposed that a supporting substrate such as alumina be bonded between a printing element substrate and a supporting member, thereby preventing the breakage of the printing element substrate due to a difference in the coefficient of linear expansion.
In recent years, in order to reduce the cost of a printing element substrate which is the most expensive among production costs of an inkjet printing head (hereinafter, simply referred to as a printing head as well), there have been many requests that the printing element substrate be downsized to increase the number of printing element substrates per silicon wafer. It has been considered that ejection port rows be arranged in narrower intervals as a means of downsizing the printing element substrate. However, arrangement of the ejection port rows in narrower intervals will always entail a thinner wall part on the periphery of an ejection port, thus resulting in a less stiff part on the printing element substrate. As described above, since the periphery of the printing element substrate is sealed by a thermosetting-type sealant, shrinkage on curing will generate stress inside the sealant, and the stress acts so as to draw the printing element substrate outwardly.
FIG. 11 is an enlarged view of a peripheral part of the sealant 805 in the cross sectional view of FIG. 10, and shows how the stress inside the sealant 805 acts on the printing element substrate 702. In production of the printing head, in order to cure an adhesive agent and the sealant for fixing the printing element substrate 702, the supporting plate 804 and the supporting substrate 802, these are placed into an oven kept at 100° C. In this case, these members undergo thermal expansion and are cured in an expanded state. A coefficient of expansion at this time is different depending on the material of each of the members. When the printing head is taken out from the oven after the adhesive agent and the sealant are cured, these members will return to their original state from an expanded state due to the fact that the temperature is lowered to a room temperature. Further, a generally-used thermosetting-type sealant is known to undergo approximately 5% shrinkage on curing. Therefore, as shown in FIG. 11, due to the shrinkage on curing and change in temperature of these members, stress is generated inside the sealant 805 in a direction indicated by the arrow in the figure. Although not shown in the figure, which only covers a part of the inkjet printing head, a similar state is found on the side face opposite to the printing element substrate 702. More specifically, there is a force working on the printing element substrate 702 from the side face to the outside.
As described above, where there is found stress on the printing element substrate 702, as apparent from FIG. 10, the sides of an opening part of the printing element substrate 702 in contact with the sealant 805 are small in cross-sectional area and, therefore, may be easily deformed where the force is applied thereto. Further, the printing element substrate 702 is different in opening area composed of ejection ports, ink supply ports and others between the face in contact with the supporting substrate 802 and the face opposing thereto. Therefore, the printing element substrate 702 may be bent backwardly and deformed, if the force is applied thereto by the sealant 805. Where the printing head of the thus deformed printing element substrate 702 is used to print, ink droplets ejected from the printing head of an inkjet printing apparatus are attached at deviated positions to deteriorate the printing quality. Further, there is a case where the printing element substrate 702 may be deformed and broken during the production.